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There are a whole bunch of parts that can't self-diagnose.
I don't know diddly about engines, and can't find the answer in my FSM. Problem is, there are many many things that can cause bad gas mileage.
Here's something I found elsewhere:
What should I do if my fuel economy is excessively low?
If your fuel economy is excessively low, you may have vehicle related problems. You may want to contact your dealer or a reputable repair facility to check your vehicle. The following are some of the diagnostic checks which a mechanic will typically conduct for poor mpg:
* Perform "On-Board Diagnostic System
* Check" for diagnostic trouble codes (sensors or actuators which may be malfunctioning)
* Check for dragging brakes
* Check transmission shift patterns for slipping, use of all gears, lock-up operation
* Check ignition timing
* Check owner's driving habits including A/C usage, hard accelerations, carrying heavy loads
* Check emission control system
* Check vacuum hoses for leaks, kinks, proper routing
* Check tire pressure
* Check fuel type, quality, and alcohol content
* Check fuel pressure and fuel system for leaks
* Check air cleaner element
* Check coolant level
* Check ignition system for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits, cracking or improperly connected ignition wires.
* Check for proper calibration of speedometer
* In extreme cases, there may be engine problems such as poor compression or faulty fuel injectors
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